HVAC system with positive temperature coefficient varying along length of heat rod

ABSTRACT

In a first aspect, a vehicle HVAC system includes: a housing that defines at least first and second air conduits to a vehicle interior compartment; and at least first and second heat rods that each traverses the first and second air conduits, wherein a first positive temperature coefficient along a length of the first heat rod is greater at the first air conduit than at the second air conduit, and wherein a second positive temperature coefficient along a length of the second heat rod is greater at the second air conduit than at the first air conduit.

CROSS REFERENCE TO RELATED APPLICATIONS

The present U.S. Utility Patent Application claims priority pursuant to35 U.S.C. § 120 as a continuation of U.S. Utility application Ser. No.14/017,170 entitled “HVAC SYSTEM WITH POSITIVE TEMPERATURE COEFFICIENTVARYING ALONG LENGTH OF HEAT ROD”, filed Sep. 3, 2013, which is herebyincorporated herein by reference in its entirety and made part of thepresent U.S. Utility Patent Application for all purposes.

BACKGROUND

Most vehicles that have an interior compartment (sometimes referred toas a passenger compartment) provide some form of heating and/orventilation for the person(s) traveling inside the vehicle. For example,a heating, ventilation and air conditioning (HVAC) system is sometimesprovided.

Some HVAC systems are designed to provide multi-zone climate control.For example, different heating (or cooling) settings can be made for thedriver and front-seat passenger, respectively. Climate settings cansometimes be differentiated between the respective rows of seating inthe vehicle, for examples to provide second-row (or third-row)passengers increased control over the temperature or other climateaspects in their respective area.

Differentiated temperature settings can pose particular challenges forHVAC equipment. For example, some systems have electric heating rodsthat traverse the respective conduits for different climate zones, andthe differentiated temperature settings are then metered by restricting(or blocking) the air flow in one of the conduits (i.e., the one withthe cooler setting). However, the heat rod(s) that the system energizesto provide heat for the conduit with the hotter setting can overheat dueto the lack of air flow.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an example HVAC system including an air heater.

FIG. 2 is a partial cross section showing an example of the air heaterfrom FIG. 1.

FIG. 3 shows an example of the air heater from FIG. 1 wherein a dividerseparates the respective air conduits.

FIG. 4 shows an example of a heat rod having a biased positivetemperature coefficient.

DETAILED DESCRIPTION

This document describes systems and techniques that improve heating ofinterior vehicle compartments, particularly the ability to providesubstantially different temperatures in different areas of the vehicleusing electrically heated resistive elements. In some implementations, asingle heater consists of several heating rods that are spaced apartfrom each other and connected by interleaving aluminum fins to dissipatethe heat from the heating rods to the fluid stream. The resistiveelement in the heating rod is a ceramic element which has a positivethermal coefficient (PTC) such that the electrical resistance of theceramic element increases with its temperature. The PTC property of theceramic element provides a self-limiting temperature at which noadditional power can be dissipated through the element and thetemperature of the element remains constant. The resistance versustemperature function of the ceramic element can be specificallyestablished by adjusting the physical properties of the ceramic materialand this allows for establishing a specific limiting temperature of theceramic.

In some implementations, an individual heating rod consists of multipleceramic elements, some of which have a higher self limiting temperature,and thus a higher output temperature, than the others. For example, thehigh output ceramic elements can be grouped on one half of the heatingrod, and the lower output ceramic elements can be grouped on the otherhalf of the heating rod.

In some implementations, each rod is controlled individually. In otherimplementations, two or more rods can be controlled by the same powersource device.

In some implementations, a single heater having multiple heating rods isused to provide heat to multiple air outlets. For example, the heat rodscan be transversely mounted across respective air conduits to the driverand passenger sides. To heat the driver side significantly more than thepassenger side, the system energizes a heat rod that has its PTC biasedtoward the driver side by having the higher output ceramic elements onthe driver side of the energized heating rod. Similarly, to heat thepassenger side significantly more than the driver side, the systemenergizes another heat rod that has its PTC biased toward the passengerside by having the higher output ceramic elements on the passenger sideof the energized heating rod.

FIG. 1 shows an example HVAC system 100 including an air heater 102. Insome implementations, the HVAC system is configured to operate inresponse to climate control settings generated either by direct userinput (e.g., the user sets a specific temperature) or by a controlmechanism in the vehicle (e.g., the vehicle has predefined programmaticsettings for HVAC equipment). For example, the HVAC system can regulateone or more climate aspects of an interior compartment, including, butnot limited to, temperature, air flow, or humidity.

In the illustrated example, the HVAC system 100 has a first heating side104A and a second heating side 104B, and is therefore capable ofproviding climate control in two separate zones. For example, the firstheating side 104A can be directed to a driver's seat and the secondheating side 104B can be directed to one or more passenger seats.Advantageously, the air heater 102 can be used also when the temperaturesettings between climate zones are significantly different from eachother.

FIG. 2 is a partial cross section showing an example of the air heater102 from FIG. 1. Particularly, a portion of the housing of the HVACsystem 100 has been omitted to show the inside of the first heating side104A. The heating side 104B, in contrast, is shown as in the previousfigure. The air heater 102 bridges the heating sides 104A-B. The airthat flows past the air heater on the first heating side 104A continuesinto a first duct 200A (e.g., a driver side duct), and the air thatflows past the air heater on the second heating side 104B continues intoa second duct 200B (e.g., a passenger side duct). The air heaterincludes two or more heat rods, each of which traverses the respectiveair conduits formed by the heating sides 104A-B. By selectivelyenergizing one or more specific heat rods, a temperature differentialcan be created between the two climate zones.

In some implementations, the HVAC system 100 also includes at least oneflue 202 that can be used to reduce air flow through either air conduit.Here, the flue is located on the first heating side 104A and is shown inan open position; that is, the flue currently does not restrict air flowon the first heating side or through the first duct 200A. In somesituations, the flue can be used to limit the air flow through thatheater side, but the air heater could become overheated if not enoughair passes through it.

FIG. 3 shows an example of the air heater 102 from FIG. 1 wherein adivider 300 separates the respective air conduits. For example, withreference briefly again to FIG. 2, the divider can be comprised of awall between the first and second heating sides 104A-B that serves todefine the respective air conduits to the first and second ducts 200A-B.In this example, the climate zones will be referred to as a driver side302A and a passenger side 302B for simplicity, with the understandingthat other climate zones can be defined in some implementations.

Here, the air heater 102 has nine heat rods 304. Each of the heat rodstraverses the driver side 302A and the passenger side 302B, in thisexample at a right angle to the divider 300. Each of the heat rods isconfigured to generate heat when electrically energized. In someimplementations, current flow through the heat rods can be controlledusing switches 306. For example, the switches can compriseinsulated-gate bipolar transistors (IGBTs). The switches can be cycledon and off during operation, to name just one example.

The second heat rod is here labeled 304A and the eighth heat rod is herelabeled 304B. Each of the heat rods 304A-B has a biased PTC. Here, theheat rod 304A is considered to be biased toward the passenger side 302B,and the heat rod 304B is considered to be biased toward the driver side302A.

The PTC bias is a result of the heat rod having different materialproperties along its length. For example, a first portion 308A of theheat rod 304B is here made of a material that has a relatively highthermal output, and a second portion 308B of the heat rod 304B is heremade of a material that has a relatively low thermal output. That is,the PTC along the length of the rod 304B is greater at the first portion308A than at the second portion 308B.

When current flows through the heat rod 304B, more thermal energy willbe generated by the first portion 308A (i.e., in the driver side 302A)than by the second portion 308B (i.e., in the passenger side 302B). Inother words, energizing the heat rod 304B leads to more heat flowing tothe driver side than to the passenger side. That is, when trying to keepthe driver side hot and the passenger side cool, it is advantageous torun current through the heat rod 304B, because it delivers more thermalenergy on the driver side. In other implementations, two or more heatrods can be biased toward the driver side.

The heat rod 304A, in turn, is PTC biased in the opposite direction. Thematerial with relatively high thermal output is here on the passengerside 302B and the material with relatively low thermal output is here onthe driver side 302A. That is, the PTC operating temperature greater onthe passenger side than on the driver side. In other words, the heat rod304A should be energized when trying to keep the passenger side hot andthe driver side cool, because it delivers more thermal energy on thepassenger side. In other implementations, two or more heat rods can bebiased toward the passenger side.

Examples of the heat rods 304A-B are described below. The other heatrods 304—i.e., heat rods one, three, four, five, six, seven and nine—canbe unbiased. That is, in these heat rods the thermal output can besubstantially even throughout the length of the rod, for a givencurrent. Accordingly, one or more of these heat rods can be energized(or de-energized) to increase (or decrease) the overall amount of heatavailable when a temperature differential is required.

Each of the switches 306 can control current through one or more of theheat rods. In some implementations, when six IGBTs are used, they cancontrol individual heat rods as follows:

IGBT 1 2 3 4 5 6 Heat rod 2 8 5 6 1 3 number Heat rod 4 7 9 number

For example, when the fifth IGBT is switched on it allows current toflow through the heat rods one and seven.

Various combinations of the IGBTs can be switched on to providedifferent temperature settings with or without a PTC bias, for exampleas follows:

Combination IGBT(s) On Heat rod(s) On Bias? A 1 Two Passenger B 2 EightDriver C 1, 2, 3 Two, five, eight — D 1, 2, 3, 4 Two, four, five, — six,eight E 1, 2, 3, 4, 5 One, two, four, — five, six, seven, eight F AllAll —

FIG. 4 shows an example of a heat rod 400 having a biased positivetemperature coefficient. Here, the heat rod comprises eight stones,sometimes referred to as “PTC thermistors” or “PTC-resistors,” or simply“PTC stones”. Some of the stones have different thermal properties fromeach other. For example, stones 402 are here made of a material that hasa relatively high thermal output, and stones 404 are here made of amaterial that has a relatively low thermal output. In this example, thestones 402 abut the stones 404 in the middle (e.g., at the divider). Inother implementations, different combinations of stone materials, suchas unequal numbers of stones, can be used.

PTC thermistors are doped polycrystalline ceramic on a base of bariumtitanate (BaTiO₃). Selected doping produces a desired high electricalconductivity of this material at low temperatures.

In operation, an HVAC system having two or more PTC-biased heat rods canbe used to provide differentiated temperature between climate zones.Assume, for example, that the driver of the vehicle prefers a coolenvironment and therefore sets the temperature for the driver siderelatively low. The driver does this by entering a temperature value(e.g., as a number on a Fahrenheit scale or on an arbitrary temperaturescale) into the vehicle's climate control equipment.

Assume, moreover, that the front passenger of the vehicle prefers a hotenvironment and therefore sets the temperature for the passenger siderelatively high. The passenger enters a corresponding temperature valuein the vehicle's climate control equipment.

Based on these different temperature settings, the vehicle will use oneor more heat rods in the HVAC system. For example, if the driver sidetemperature should be higher than the passenger side temperature, a heatrod biased toward the driver side can be energized, and vice versa.

The amount of heat generated by the heat rod can depend on the durationin which it is cycled on. For example, a longer duration of powercycling can produce a higher overall output from the heat rod.

A number of implementations have been described as examples.Nevertheless, other implementations are covered by the following claims.

What is claimed is:
 1. A vehicle HVAC system comprising: a housing thatdefines at least first and second air conduits to a vehicle interiorcompartment; and at least first and second heat rods that each traversethe first and second air conduits, wherein a first positive temperaturecoefficient along a length of the first heat rod is greater at the firstair conduit than at the second air conduit, and wherein a secondpositive temperature coefficient along a length of the second heat rodis greater at the second air conduit than at the first air conduit. 2.The vehicle HVAC system of claim 1, wherein the first air conduit leadsto a driver position, and the second air conduit leads to a passengerposition.
 3. The vehicle HVAC system of claim 1, wherein each of thefirst and second heat rods comprises different stones along its length.4. The vehicle HVAC system of claim 3, wherein the different stones abuteach other at a divider between the first and second air conduits. 5.The vehicle HVAC system of claim 4, wherein stones on one half of thefirst heat rod have a higher positive temperature coefficient thanstones on another half of the first heat rod.
 6. The vehicle HVAC systemof claim 1, wherein each of the first and second heat rods traverses thefirst and second air conduits at a right angle with a divider betweenthe first and second air conduits.
 7. The vehicle HVAC system of claim1, further comprising at least a third heat rod.
 8. The vehicle HVACsystem of claim 7, wherein a third positive temperature coefficientalong a length of the third heat rod does not vary between the first andsecond air conduits.
 9. The vehicle HVAC system of claim 7, wherein eachof the first, second and third heat rods is controlled individually. 10.The vehicle HVAC system of claim 1, wherein each of the first and secondheat rods is controlled individually.
 11. The vehicle HVAC system ofclaim 1, wherein: wherein the first heat rod has different materialproperties along its length such that a first portion of the first heatrod has a first positive thermal coefficient and a second portion of thefirst heat rod has a second positive thermal coefficient; and whereinthe second heat rod has different material properties along its lengthsuch that a first portion of the second heat rod has a third positivethermal coefficient and a second portion of the second heat rod has afourth positive thermal coefficient.
 12. The vehicle HVAC system ofclaim 1, wherein the first heat rod and the second heat rod aresubstantially parallel to one another.
 13. The vehicle HVAC system ofclaim 1, wherein each of a first portion of the first heat rod and afirst portion of the second heat rod traverses the first air conduit anda second portion of the first heat rod and a second portion of thesecond heat rod traverse the second air conduit.